In electrophotography, it is usually practiced to forming an electrostatic latent image by charge and exposure to light, on a latent image carrier comprising a photoconductive photosensitive member, subsequently developing this latent image by use of a toner, and transferring the resulting toner image to a support such as transfer paper, followed by fixing the toner under heating or pressure.
Formation of a visible image formed through such an electrostatic latent image should preferably be achieved at high speed, and hitherto widely employed from this viewpoint has been a heated roller fixing system that can attain high thermal efficiency in the fixing process and can be more advantageous as compared with other systems.
Recently, however, it is strongly desired to achieve further higher speed processing, and a necessary condition for that purpose is to carry out the fixing of a toner at high speed.
Accordingly, it is necessary for a toner to have the properties or performances as shown below.
(1) To have a good triboelectric property. PA0 (2) To have a good thermal fixing performance. PA0 (1) A cross linked polyester resin comprising linear polyesters obtained from a diol component containing a particular etherified bisphenol and a monomer component containing an aromatic dicarboxylic acid, cross-linked by a polybasic carboxylic acid having three or more carboxylic acid groups (proportion in the acid component: 30 to 80 mol %). (See Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O. P. I. Publication) No. 37353/1982). PA0 (2) A cross-linked polyester resin comprising linear polyesters obtained from a monomer component containing a particular etherified bisphenol (proportion of propoxy groups: 50 mol % or more) and phthalic acids, cross-linked with an aromatic carboxylic acid having 3 or more carboxylic acid groups (proportion of content in the acid component: 40 mol % or less). (See Japanese Patent O.P.I. publication No. 29255/1984.) PA0 (1) Can attain a sufficient triboelectric property, and, moreover, has a small change in the electrical charge quantity against humidity, and small environment dependency of the image density. PA0 (2) Has an excellent fixing performance at low temperature, and enables high speed fixing. PA0 (3) Can have a sufficient anti-offset property. PA0 a step of forming imagewise an electrostatic latent image on a photoconductive element; PA0 a step of developing said latent image with a color toner bearing an electrostatic charge thereon, said color toner comprising a polyester resin obtained by polymerizing by condensation polymerization PA0 a step of transferring said developed image to a recording sheet; and PA0 a step of bringing said toner image formed on the recording sheet into contact With a heated roller under pressure. PA0 C.I. Pigment Red 5 PA0 C.I. Pigment Red 48:1 PA0 C.I Pigment Red 53:1 PA0 C.I. Pigment Red 57:1 PA0 C.I. Pigment Red 123 PA0 C.I. Pigment Red 139 PA0 C.I. Pigment Red 144 PA0 C.I. Pigment Red 149 PA0 C.I. Pigment Red 166 PA0 C.I. Pigment Red 177 PA0 C.I. Pigment Red 178 PA0 C.I. Pigment Red 222 PA0 C.I. Pigment Orange 31 PA0 C.I. Pigment Orange 43 PA0 C.I. Pigment Yellow 17 PA0 C.I. Pigment yellow 14 PA0 C.I. Pigment yellow 138 PA0 C.I. Pigment yellow 93 PA0 C.I. Pigment yellow 94 PA0 C.I. Pigment Green 7 PA0 C.I. Pigment Green 36 PA0 C.I. Pigment Blue 15:3 PA0 C.I. Pigment Blue 60 PA0 C.I. Pigment Violet 23 PA0 "Viscol 660P" (available from Sanyo Chemical Industries, Ltd.) PA0 "Viscol 550P" (available from Sanyo Chemical Industries, Ltd.) PA0 "Polyethylene 6A" (available from Allied Chemical Corp.) PA0 "Hi-wax 400P" (available from Mitsui Petrochemical Industries, Ltd ) PA0 "Hi-wax 100P" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hi-wax 200P" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hi-wax 320P" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hi-wax 220P" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hi-wax 2203A" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hi-wax 4202 E" (available from Mitsui Petrochemical Industries, Ltd.) PA0 "Hoechst Wax PE 520" (available from Hoechst Japan Co.) PA0 "Hoechst Wax PE 130" (available from Hoechst Japan Co.) PA0 "Hoechst Wax PE 190" (available from Hoechst Japan Co.)
More specifically, in order to develop an electrostatic latent image in a good state, it is necessary for a toner to show an appropriate polarity by triboelectric charge and yet to retain electrical charges in a charge quantity of an appropriate range.
That is as follows: The heated roller fixing system can be preferably used as a means for fixing a toner. Since however in this heated roller fixing system the fixing is usually carried out by bringing a heated roller into contact with a toner, there is a tendency to cause the so called offset phenomenon that part of the toner which forms an image at the time of the fixing may be transferred to the surface of the heated roller when the temperature of the heated roller became higher, and this part may be again transferred to the image support such as transfer paper subsequently fed, to stain the image. Accordingly, in order to carry out the fixing in a good state, the minimum temperature at which the offset phenomenon may occur (hereinafter "offset occurring temperature") is required to be higher. Also, when the fixing of a toner is carried out at high speed according to the heated roller fixing system, the contact time of the toner with the heated roller is short, and the heat tends to be absorbed to the image support such as transfer paper to lower the temperature of the heated roller, whereby poor toner fixing may readily occur. Therefore, in order to prevent such poor fixing, the toner is required to be the one that can be sufficiently fixed even at lower temperature.
And, in the so-called black toner, it is relatively easy to control the triboelectric property of the toner since the carbon black usually used as a coloring agent has the charge controlling action and any charge controlling agent having a specific color can be used almost without any limitation.
However, in a color toner in which sharp coloration is required, it is generally not preferable to use a charge controlling agent having a specific color that may impair the color tone of a toner, and it is in practice considerably difficult to control the triboelectric property of the toner by use a pigment or dye usually used as a chromatic coloring agent. Accordingly, there are may limitations in selecting the charge controlling agent, and it is not necessarily easy to control the triboelectric property of the toner.
Also, in the case of the color toner, it is generally difficult for the chromatic coloring agent to be formed into fine particles of small particle size by grinding as compared with the carbon black in the black toner. Moreover, since the chromatic coloring agent has small compatibility with a binder, it tends to have insufficient dispersibility to its binder. As a result, it is difficult to obtain sufficient color density and opecifying ability.
To cope with this, it can be contemplated that the proportion of the content of the chromatic coloring agent in a toner may be made large to increase the color density and opecifying ability. By doing so, however, it follows that coloring particles having large particle size are present in toner particles in a high density. As a result, problems may arise such that not only the toner particles are readily broken, but also the softening point of the toner is increased to raise the minimum fixing temperature, and also the elasticity of a toner at the time of fusing is lowered to readily cause the offset phenomenon.
Thus, the state of things in the color toner is that there may be accompanied with many limitations in selecting resins used as the binder.
Also, recently has been disclosed a technique in which the resins as shown below are used as a binder of a toner.
In the resin of the above (1), however, since the proportion of the polybasic carboxylic acid having three or more carboxylic acid groups is 30 mol % or more, there may remain many unreacted carboxylic groups, whereby the resin may be readily affected by water content, and, as a result, in the toner the quantity of triboelectric charge may greatly change depending on humidity, to produce a problem that the image density may have a large environment dependency.
Also, in any of the resins of the above (1) and (2), since the aromatic carboxylic acid is used as a carboxylic acid having two carboxylic acid groups, the glass transition point of the resin is so high that, although no problem may be caused when the pressure by the heated roller is large and the contact time with the heated roller is long in the heated roller fixing system, there may be involved in a problem that the fixing performance is extremely worsened when the pressure by the heated roller is small and the contact time wIth the heated roller is short as in the case where the fixing is carried out at high speed.